Broad long-term objective: To define the photobiological role of melanin in the cause and prevention of malignant melanoma and other skin cancers. To this end, the experimental materials will be 1.) mouse melanoma cell lines of varying, tunable melanization potential and 2.) human melanocytes and acquired and congenital nevi - presumptive malignant melanoma precursors - cultured under conditions which a.) stimulate melanin synthesis or b). allow for its loss. Specific aims: 1.) Using the mouse lines, to define the role of melanin in survival and mutagenesis by irradiating, cells of differing melanin content with near monochromatic light in the UVC, UVB and UVA ranges and with broad band solar light. 2.) Using the same mouse lines to define the role of melanin in DNA damage induction by the same radiations by measuring damage of 2 specific types: a.) direct photon damage which will be quantitated using an anti-thymine dimer antibody and by UV-endonuclease-sensitive sites together with alkaline gel electrophoresis and b.) indirect damage of the type caused by oxygen stress and photosensitizations which will be quantitated using an antithymine glycol antibody and by endonuclease III-sensitive sites together with alkaline gel electrophoresis. 3.) Using several human lines with high and low melanin content to replicate the experiments of specific aim #2. 4a.) Using both the mouse and the human cell lines, to determine the levels of radicalscavenging enzyme systems at the time of irradiation to determine their potential role in the pathobiological responses of the various cells and their correlation with melanization; b.) to assess the rate of removal of specific lesions in cells varying n melanin content after irradiations of the various types. Health-relatedness of the project: Malignment melanoma is one of the most rapidly increasing cancers. It has been doubling every 10 years for the past several decades. The projected decrease in stratospheric ozone may exacerbate this already significant increase. Although it is clear that rates of melanoma and skin cancer are reduced in dark skin, the dogma that melanin acts as an inert sunscreen must be questioned in the light of increasing evidence that melanin is photobiologically very reactive. Although it can scavenge free radicals and active oxygen species. It can also produce such species - including hydroxvl radical - upon irradiation. These studies are based on the hypothesis that melanin is a two-edged sword: it can both act as a sunscreen and a scavenger of radicals at low rates of photon energy input, but it can also produce toxic radicals that can damage DNA at high rates of photon energy input.